1. Field of the Invention
The present invention relates to a system for determining the position of a vehicle using two or more satellites.
2. Description of the Prior Art
A satellite-based system for determining the position of a vehicle and for communication service systems for use by moving objects such as vehicles, airplanes, ships, or the like which uses two or more artificial satellites and which can also perform message communication is being developed. Such a system is disclosed in U.S. Pat. No. 4,359,733, in which a transponder is carried by a moving object (hereinafter, simply referred to as a vehicle) and message communication between an operation center serving as a ground station and each vehicle is provided. Each transponder transmits a responsive signal to an interrogation signal from the operation center via each satellite. Thus, the operation center can determine the position of the vehicle on the basis of the time differences between the propagation times of the respective signals.
Such a satellite-based vehicle position determining and communicating service system can be used, for example, to manage movement of trucks in a transport company. For example, when using this system to manage the movement of trucks, transponders are attached to each of the trucks which are operated by the transport company. The trucks are distributed in every district and are moving and their positions constantly change. Message communication is also performed between the operation center and each truck. The operation center can monitor the velocity and present position of each truck. According to this system, the transport company can perform services such that a customer is informed of the time of arrival of a parcel, for example. On the other hand, the next schedule of moving a truck can also be planned, so that the trucks can be efficiently operated.
Such satellite-based vehicle position determining and communicating service systems include not only a one-way communicating service in which a message is unidirectionally communicated from a vehicle to the operation center but also a two-way communicating service in which a message is bidirectionally communicated between a vehicle and the operation center.
In the two-way communicating service, the position of a vehicle can be determined and at the same time, message communication can be performed between the vehicle and the operation center. Therefore, in the two-way communication service, an instruction for movement can be also given from the operation center to a vehicle.
FIG. 1 is a conceptional diagram for explaining such a vehicle position determining system which uses satellites. The system in this diagram can obtain the position of each vehicle in three dimensions by using three satellites. A satellite 201 is a geostationary satellite for transmitting a signal from a vehicle 206 to a ground station 205 and for transmitting a signal from the ground station 205 to the vehicle 206. Satellites 202 and 203 are geostationary satellites for transmitting the signal from the vehicle 206 to the ground station 205.
When the position of the vehicle 206 is to be determined, an interrogation signal from the ground station 205 is sent to the vehicle 206 through the satellite 201. A transponder mounted in the vehicle 206 receives the interrogation signal from the ground station 205 and sends an output response signal in responsive to the interrogation signal. The response signal is received by the ground station 205 through the satellites 201, 202, and 203. In the ground station 205, the position of the vehicle 206 is calculated from the time differences of the response signals received from the satellites 201 to 203. The vehicle position is calculated on the basis of triangulation.
It is assumed that the distances from the ground station 205 to the satellites 201, 202, and 203 are respectively l.sub.201, l.sub.202, and l.sub.203 and the distances from the satellites 201, 202, and 203 to the vehicle 206 are l.sub.210, l.sub.220, and l.sub.230, respectively. An interrogation signal is transmitted from the ground station 205. It is assumed that the times when the response signal generated from the transponder of the vehicle in response to the interrogation signal reaches the ground station 205 through the satellites 201, 202, and 203 are t.sub.201, t.sub.202 and t.sub.203, respectively. In this case, the following equations are satisfied (where, C denotes the velocity of light). ##EQU1## Since the distances l.sub.201, l.sub.202, and l.sub.203 are already known, the distances l.sub.210, l.sub.220, and l.sub.230 can be obtained if the times t.sub.201, t.sub.202, and t.sub.203 are known. Thus, the position of the vehicle 206 can be three-dimensionally obtained.
On the other hand, the position of a vehicle can be also measured by using two satellites, e.g., the satellites 201 and 202. In this case, the vehicle position can be two-dimensionally measured. The measurement is performed on the assumption that one of three coordinates of latitude, longitude, and altitude, e.g., the altitude are known.
In the case for determining the vehicle position by use of, e.g., three satellites 201 to 203 on the basis of the time differences of the interrogation signal from the ground station 205 has been output until the response signals responsive to the interrogation signal are received from the transponder of the vehicle 206 through the satellites 201 to 203, respectively, if the times t.sub.201, t.sub.202, and t.sub.203 until the response signals return to the ground station through the satellites 201, 202 and 203 are not accurately measured in the ground station, a position measurement error occurs. To prevent this, in the transponder, the response signal which is output in response to the interrogation signal from the ground station has to be completely time synchronized with the interrogation signal. However, a time lag of the response signal to the interrogation signal actually has a particular value in each transponder. For example, assuming that the time lag until the transponder transmits a response signal after it receives the interrogation signal deviates from a preset value by 10 nsec, a position measurement error of 3 m will result.
Therefore, in a conventional satellite-based vehicle position determining and communicating service system, a high speed device such as an ECL (emitter coupled logic) or the like is used as a circuit device for the transponder in the vehicle 206 or a time adjusting circuit is interposed, thereby allowing the time synchronization between the interrogation signal and the response signal in each transponder. However, the use of such high speed device causes the circuit size and cost to be increased. In addition, even if such a high speed device is used, it is difficult to make the time synchronization perfect between the interrogation signal and the response signal, so that variations in time lags of the transponders occur. Further, it is also difficult to perform adjustments so as to make the time lags of the transponders to coincide.